This invention relates to an exposure method and apparatus and, more particularly, to an exposure method and apparatus for exposing a substrate to a fine circuit pattern. An exposure method and apparatus of this kind find use in the manufacture of semiconductor chips such as IC and LSI chips and various devices such as display elements used in liquid crystal panels and the like, detection elements used in magnetic heads and the like, and image sensing elements such as CCDS.
When an IC or LSI chip or a device such as a liquid crystal panel is manufactured using photolithography, the projection exposure apparatus currently used most generally employs an excimer laser as the light source. However, with a projection exposure apparatus using an excimer laser as the light source, it is difficult to form a fine pattern having a line width of less than 0.15 xcexcm.
In theory, the NA (numerical aperture) of the projection optical system should be enlarged or the wavelength of the exposing light reduced in order to raise the resolution of the exposure apparatus. However, this is not easy to achieve. Specifically, since the depth of field of the projection optical system is inversely proportional to the square of the NA and proportional to the wavelength xcex, enlarging the NA of the projection optical system diminishes the depth of field, makes focusing difficult and lowers productivity. Further, the transmittance of most glass materials is extremely low in the far ultraviolet region. For example, at wavelengths of less than 193 nm, transmittance falls to almost zero even in the case of fused quartz, which is used at a wavelength of 248 nm (in a KrF excimer laser). At present, there is not a glass material available that can be used practically in a region of exposure wavelengths of less than 150 nm, which corresponds to a fine pattern having a line width of less than 0.15 xcexcm obtained by ordinary exposure.
There has been proposed an exposure method of higher resolution obtained by subjecting a substrate to double exposure, namely to two-beam interference exposure and ordinary exposure, and applying a multivalued exposure distribution to the substrate at this time. In accordance with this method, a pattern with a minimum line width of 0.10 xcexcm can be formed using a projection exposure apparatus having an exposure wavelength of 248 nm (using a KrF excimer laser) and an NA of 0.6 on the image side of the projection optical system.
According to the exposure method mentioned above, two-beam interference exposure is performed by exposure with so-called coherent illumination using an L and S (line and space) phase-shift mask (or reticle) having a line width of 0.1 xcexcm, after which ordinary exposure (e.g., exposure by partially coherent illumination) is carried out using a mask (or reticle) formed to have a real element pattern of a minimum line width of 0.1 xcexcm. Thus, with the double-exposure method, two exposure steps involving different exposure information are required for each shot in order to form one pattern. A problem that arises is reduced throughput.
An object of the present invention is to raise the throughput of a multiple-exposure method in which one type of pattern is formed by superimposing and printing patterns of a plurality of types on the same shot of a substrate to be exposed.
A substrate to be exposed is exposed to the pattern image of a reticle by synchronizing the reticle and the substrate and scanning them relative to a slit-shaped beam while part of the pattern image of the reticle is illuminated in the shape of the slit and projected upon the substrate. In order to attain the object set forth above, the present invention is such that a plurality of patterns are formed on the reticle beforehand so as to be arrayed in the scanning direction, and illuminating conditions or focus target values are changed over at vacant spaces between patterns during scanning exposure.
In accordance with the present invention, patterns of a plurality of types are formed on a single reticle and a substrate is exposed to these patterns by a single scan in the direction in which the patterns are arrayed. The result is higher throughput. Since the exposure conditions are changed over at the vacant spaces between patterns, exposure performance is improved.
In a preferred embodiment of the present invention, the wavelength of the exposing light and projection magnification of the pattern images are changed in conformity with the focus target value. Further, the vacant space between the patterns on the reticle are made broader than an ordinary scribe line to afford a greater temporal margin for the changeover. Furthermore, by partially overlapping scanning exposure zones and repeating exposure, the plurality of patterns are projected onto the same exposure position of the substrate in a superimposed form to thereby expose the substrate at this position.
The changeover of illumination conditions is performed by providing a plurality of illumination systems in which light beams, which have been obtained by splitting a beam emitted from a single exposure light source, are set to respective ones of desired illumination conditions, and illuminating each pattern by the respective illumination system. In other words, if a pattern illuminated by a slit is changed over, the illumination conditions also change over automatically. As a result, it is possible to prevent a decline in throughput brought about by changing over the illumination conditions of an illumination system in dependence upon the pattern. Throughput can thus be raised.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.